The study of tumours inoculated into the brain is of interest for several reasons. In the first place, the histologic structure of the nervous system permits sharp differentiation of its own elements from the components of the neoplasm, so that the process of tumour growth, as well as the reaction which this excites in the host tissue, can be interpreted with precision. The normal and pathological features of the various cellular constituents of nervous tissue are sufficiently well known to allow some biological characteristics of a neoplasm to be deduced from a study of the changes resulting from its growth. Also, the physico-chemical properties of the nervous tissue and its abundance of blood vessels permit the ready development of all kinds of grafts from normal or pathological tissues. The first studies on this question were those of Da Fano (1), who made intracerebral transplantations of different strains of tumours in rats, mice, and dogs. The transplants developed readily, but at a slower rate than in the usual sites of inoculation. The tumours spread through the brain, sometimes throughout the perivascular spaces and sometimes as diffuse infiltrations. Da Fano studied, also, the reaction of the cellular elements of the brain tissue, and noted the hypertrophy of the neuroglia and the absence of a mesodermal reaction.